RESEARCH

Middle landscape evolution of southwest Laurentia using detrital zircon geochronology

Sally L. Potter-McIntyre1, Marisa Boraas2, Keegan DePriest2, and Andres Aslan2 1DEPARTMENT OF GEOLOGY, SOUTHERN ILLINOIS UNIVERSITY, PARKINSON LABORATORY, MAIL CODE 4324, CARBONDALE, ILLINOIS 62902, USA 2PHYSICAL AND ENVIRONMENTAL SCIENCES, MESA UNIVERSITY, 1100 NORTH AVENUE, GRAND JUNCTION, COLORADO 81501, USA

ABSTRACT

The Wanakah Formation in western Colorado is a poorly understood unit in terms of depositional environment and abso- lute age of deposition; however, a refined interpretation of the has important implications for understanding the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. This study presents the first U/Pb age dating of detrital zircons from the Middle to Entrada , Wanakah Formation, and Tidwell and Salt Wash Members of the . Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the ) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the McClure Mountain syenite on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Forma- tion and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 is likely not pres- ent in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across south- western Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment.

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INTRODUCTION widely exposed on the Colorado Plateau (Figs. rado, the type section includes an 18-m-thick 1 and 2). basal limestone member (The Pony Express The Colorado Plateau of the western United The Middle Jurassic Wanakah Formation is Limestone) that is not present north of the west- States in North America was situated in south- exposed throughout western Colorado and ex- ern Black Canyon of the Gunnison (O’Sullivan, west Laurentia during the Mesozoic rifting of tends south into ; stratigraphically, 1992, 2004; O’Sullivan et al., 2006). In the Black Pangea and the opening of the Gulf of Mexico the unit is above the eolian Entrada Sandstone Canyon of the Gunnison, the Wanakah Forma- (Kocurek and Dott, 1983; Blakey, 1994, 2008; and below the hypersaline lacustrine Tidwell tion contains 3–7-m-thick gypsum lenses, also Blakey and Ranney, 2008; Miall and Blakey, Member in western Colorado (Fig. 1; Peterson, not present at any other exposure (O’Sullivan, 2008). The basin was arid and centered around 1994; O’Sullivan, 2004). A regional uncon- 1992, 2004; O’Sullivan et al., 2006). 30°N. The opening of the Gulf of Mexico pro- formity (known as the J-5) is typically placed The paleogeography is currently recon- vided abundant sediments that were dispersed between the Wanakah Formation and the over- structed largely from depositional environment via paleodrainages to the NW from the lying Tidwell Member and is delineated by a and provenance studies on the Colorado Pla- Grenville and Ouachita orogens in the central regional sandstone bed called “Marker Bed A” teau (Kocurek and Dott, 1983; Peterson, 1988b; Texas uplift (Lawton and McMillan, 1999; (Fig. 1; Peterson, 1988a, 1994; Turner and Pe- Blakey, 1994; Dickinson and Gehrels, 2003, Dickinson and Lawton, 2001; Mickus et al., terson, 1992, 2004; O’Sullivan, 2004). 2010; DeCelles, 2004; Demko et al., 2004). Re- 2009). In the Early to Middle Jurassic, eolian Controversies exist over Wanakah Forma- cently, advances in U/Pb age dating of detrital deposition dominated this basin; the deposi- tion depositional environmental interpretations zircons have allowed for more refinement in tional environment then shifted to hypersa- (marine or lacustrine) and whether the unit is provenance studies and insight into the maxi- line lacustrine conditions (Peterson, 1994; even present in western Colorado (Tanner, 1970; mum ages of the Mesozoic units (Dickinson O’Sullivan, 2004). Today, these sediments Adler, 1974; Ridgley and Goldhaber, 1983; Pe- and Gehrels, 2003, 2010). This study presents constitute much of the exposures of central terson, 1988a, 1994; Kirkland et al., 1995; Turner new insight into the paleogeographic evolution western North America on the Colorado Pla- and Peterson, 1992; Anderson and Lucas, 1992, of southwest Laurentia during the Middle to teau. The Jurassic shift to a hypersaline deposi- 1994; O’Sullivan, 2004; O’Sullivan et al., 2006; Late Jurassic. Specifically, this new study uses tional environment is evidenced by the so-called Lucas et al., 2006). The Wanakah Formation is detailed analyses of petrography and stratigra- Wanakah Formation and the Tidwell Member lithologically variable and contains several dis- phy coupled with the first detrital zircon geo- of the Morrison Formation—two units that are similar members. For example, in Ouray, Colo- chronology data for the Middle Jurassic rocks

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Formation has yielded some absolute age dates from volcanic ash abundant in the formation (interpreted to be deposited from 153 to 145 6 Ma; Kowallis et al., 1998), absolute ages of the Entrada Sandstone (a Middle Jurassic eo- lianite) and the overlying Wanakah Forma­tion are not known. Several major mark the Ju- 5 rassic section on the Colorado Plateau, and this paper focuses on the J-5, which is present as an angular unconformity between the and the Tidwell Member in southern 4 at Shadscale Mesa (Fig. 2; Gilluly, 1929; 3 Bernier and Chan, 2006). The J-5 unconformity is typically extended into western Colorado and placed in between the Wanakah Formation 2 and the Tidwell Member (O’Sullivan, 2004; O’Sullivan et al., 2006) because the Sum- 1 merville Formation and the Wanakah Forma- tion are interpreted as coeval (Peterson, 1994). However, this interpreted extension of the J-5 unconformity into Colorado is controversial ~1m (O’Sullivan and Pipiringos, 1983; O’Sullivan, 2004). Herein, we investigate the presence of the J-5 unconformity and provenance of the En- Figure 1. Stratigraphy of western Colorado and Escalante Canyon study site. Black dashed lines trada Sandstone, Wanakah Formation, and the show formation boundaries. Contact between 1 and 2 is a scour surface between Entrada - Tidwell Member of the Morrison Formation. stone and Wanakah Formation. Numbers indicate sample locations for detrital zircon analysis. 1—Jebb (Entrada Sandstone); 2—Jeef (sample located above the scour surface and interpreted herein as basal Wanakah Formation); 3—2Jw (Wanakah Formation); 4—3Jw (Wanakah Formation); Purpose of Study 5—1Jmt (Marker Bed A of the Tidwell Member); 6—2Jmt (Tidwell Member). Overlying Salt Wash Member sample location is not shown. Ss—sandstone. The purpose of this study is to refine the re- gional paleogeography and landscape evolution of southwest Laurentia during Middle Jurassic on the Colorado Plateau to (1) provide new in- rek and Dott, 1983; Turner and Fishman, 1991; rifting of Pangea by evaluating the provenance sights on the landscape and drainage evolution Blakey, 2008; Blakey and Ranney, 2008; Miall and depositional environment of the Middle Ju- of the Ancestral Front Range and southwest- and Blakey, 2008). During the Middle to Late rassic section in western Colorado. We utilized ern Laurentia during the opening of the Gulf Jurassic, the depositional environments on the detrital zircon geochronology to determine the of Mexico, and (2) refine the Middle Jurassic northwest part of the Colorado Plateau were stratigraphic position of unconformities above stratigraphy, specifically, the depositional en- tidal environments (e.g., Summerville and Cur- and below the Wanakah Formation and to es- vironments and unconformities present on the tis Formations; Gilluly, 1929; Peterson, 1994; tablish provenance. The specific hypotheses eastern Colorado Plateau. Bernier and Chan, 2006). In the southeast part tested herein are: (1) A distinct and detectable of the basin (in western Colorado and New change in provenance occurred that resulted in a Geological Setting Mexico), the Wanakah Formation records a hy- change of depositional environment from eolian persaline lake system (O’Sullivan and Pipirin- (Entrada Sandstone) to hypersaline lacustrine During the Mesozoic, the subduction of the gos, 1983; Peterson, 1994; O’Sullivan, 2004). (Wanakah Formation and Tidwell Member of Farallon plate along the western edge of North The Summerville Formation in southern Utah the Morrison Formation). (2) The Entrada Sand- America created a nonmarine back-bulge basin is interpreted as coeval to the Wanakah Forma- stone, the Wanakah Formation, and the overly- between the forebulge in central Nevada to the tion (Gilluly, 1929; Peterson, 1994; Bernier ing Tidwell Member are conformable rather east and the remnant Ancestral Rocky Moun- and Chan, 2006). In the study area, the Late than separated by the regional J-5 unconformity. tains to the west that subsequently became the Jurassic Morrison Formation consists of three Colorado Plateau (Lawton, 1994). Thick pack- members: Tidwell Member, a hypersaline la- METHODS ages of fluvial, eolian and fluvio-lacustrine sedi- custrine system, Salt Wash Member, a fluvial ments were deposited into this basin throughout unit, and the Brushy Basin Member, a fluvio- Field sites were chosen for their location the Mesozoic, eroding from the Cordilleran lacustrine environment. The paleodrainages of along a west-east transect as well as for expo- magmatic arc to the west, the Appalachian oro- the Morrison Formation flow to the northwest sure quality (Fig. 2). Samples were collected for gen to the east, and the Grenville and Ouachita (Turner and Fishman, 1991; Turner and Peter- detrital zircon analysis from at Es-

orogens to the south (Dickinson and Gehrels, son, 1992; Peterson, 1994; Demko et al., 2004; calante Canyon (nsamples = 7; Figs. 1 and 2). Es- 2003, 2008, 2010). Blakey, 2008; Blakey and Ranney, 2008; Miall calante Canyon is located south of Grand Junc- A sea extended from the north onto the and Blakey, 2008). Although the relative ages tion, Colorado, at 38°42.634′N, 108°16.683′W. northern edge of the Colorado Plateau (Kocu- of these formations are clear, and the Morrison The section is accessed by heading south on

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A 39°50’N N Artists Point Grand Junction

Ribbon Trail

111°W 107°30’W

37°N

B Jurassic Outcrops Study Areas

City

Western U.S. Extent of the Colorado Plateau

Area of detail in View A

Figure 2. (A) Study area in western Colorado and southeastern Utah. (B) Map of western with study area indicated.

Route 50 from Grand Junction and heading west from the overlying Salt Wash Member (n = 1) for analytical protocol). However, from each on Road 650 past the Escalante Ranch. The sec- were also collected. Representative shale and sample, ~5–10 measurements were discarded tion from the bottom exposure of the Entrada sandstone samples were acquired for quantita- for discordance; results show the actual num- Sandstone to “Marker Bed A” of the Tidwell tive elemental mineralogy using scanning elec- ber of grains used for each sample. Data analy- Member is 15 m (for a photo of the section and tron microscopy (QEMSCAN) analyses. ses (Kolgomorov-Smirnov [K-S] tests, age the units present, see Fig. 1). Using methods defined by Gehrels et al. probability distributions, AgePick) were per- Two Entrada Sandstone samples were col- (2008) and Gehrels (2011), 105 grains were formed using the tools available from lected: one from below a prominent, regional analyzed from each sample (n = 7) at Arizona LaserChron Center. scour surface (n = 1) and one from above the LaserChron Center at the University of Ari- QEMSCAN produces mineralogical “maps” scour surface (n = 1; note that this sample is zona (see raw data in GSA Data Repository1; of samples in two dimensions using the energy- reinterpreted herein as the basal Wanakah For- and see Arizona LaserChron Center Web site dispersive X-ray spectroscopy (EDS) technol- mation; see results and discussion for details). ogy associated with a scanning electron micro- Samples from sandstones within the Wanakah 1 GSA Data Repository item 2016045, raw U/Pb age scope (SEM). A species identification protocol Formation (n = 2) were collected. Two samples dates from detrital zircons along with cumulative (SIP) in the software is used to convert elemental from the Tidwell Member (one from the basal probability plots, and measured section at Escalante abundances into mineralogical interpretations. Canyon showing lithofacies relationships, is avail- sandstone, Marker Bed A, and another from able online at www.geosociety.org/pubs​ ​/ft2016.htm, This instrument and the software were used at the second major sandstone) and one sample or on request from [email protected]. the Energy and Geoscience Institute at the Uni-

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versity of Utah. This tool essentially replaces lithofacies relationships (supplemental Fig. 1 Chukar Trail location exhibits large-scale cross- traditional point counting. For this method, six [see footnote 1]). stratification interpreted as eolian reworking of samples (from the same sandstones used for de- gypsum sand into dune deposits. trital zircon analysis, excluding the Salt Wash Wanakah Formation and Tidwell Member Member sample) were evaluated using 20 µm Mudstone exhibits three distinct lithofacies: U/Pb Age Dating of Detrital Zircons pixel spacing over an area of ~15 × 20 mm. laminated, massive, and mottled. The laminated mudstone lithofacies exhibits horizontal planar Normalized age probability spectra for RESULTS lamination with some bioturbation. This lithofa- each sample (using macros available from the cies is common throughout all sections. The mas- LaserChron Laboratory) are graphically repre- Field Observations sive mudstone lithofacies is bioturbated, and most sented with major age accumulations annotated bedding is destroyed; however, minor horizontal in Figure 4. A histogram of percentages of age In order to determine stratigraphic relations planar lamination is present. This lithofacies is populations is presented in Figure 5. Raw data and depositional environment, we constructed commonly interbedded with the siltstone lithofa- and cumulative distribution plots are available a regional characterization of the Wanakah cies and is common in all sections. The mottled as supplemental data (see footnote 1). Formation. One of the defining characteristics mudstone is bioturbated with some blocky tex- of the formation is a lower red diagenetic fa- tures (peds), root traces, and mottled color (red QEMSCAN Analysis cies and an upper green diagenetic facies (Figs. and green). Calcite concretions (<10 cm in diam- 3A and 3B). A black volcanic ash separates the eter) are common in this lithofacies. A siltstone The interesting feature of the petrographic red from the green diagenetic facies and is ob- lithofacies is also common that is horizontally analysis is the amount of K- in the sam- served at Escalante Canyon, Ribbon Trail, Art- laminated to massive and exhibits bioturbation. ples from the lower section. Data are included in ists’ Point, and Ten Mile Graben (Figs. 3C and This lithofacies is commonly interbedded with supplemental data (see footnote 1). The Entrada 3D). The unit at Ten Mile Graben is interpreted the massive mudstone and can exhibit a fining- Sandstone is primarily a (from in the literature as the Tidwell Member; how- upward into the massive mudstone lithofacies. field observations); however, there is a distinct ever, the red and green diagenetic facies sepa- Three categories of sandstone lithofacies lithological change observable with a hand lens rated by a black ash are present in this section are observed: cross-bedded, wavy, and planar in the upper facies and above the scour surface too. In the lower part of the section at Escalante stratified. The cross-bedded sandstone lithofa- (interpreted herein as the basal sandstone of the Canyon, a tabular sandstone with three distinct cies is fine- to medium-grained quartz aren- Wanakah Formation; see discussion for details). subunits is topped with microbial mats (Figs. ite (determined via hand lens in the field and QEMSCAN analysis supports this field obser- 3E and 3F). Gypsum beds are present at Chu- quantitatively with QEMSCAN, sample 2Jmt). vation. Entrada Sandstone sample 1Jeeb is a kar Trail and Duncan Trail. Feldspar and matrix content is <10%. Coarse- subarkosic arenite with 14 area % K-feldspar. A sharp transition from sandstone to shale grained chert lags are common. The unit directly above the scour surface (1Jeef; typically marks the boundary between the En- The wavy sandstone lithofacies is a fine- herein reinterpreted as basal Wanakah Forma- trada Sandstone and the Wanakah Formation; grained feldspathic arenite with <10% matrix tion; see following for discussion) is an arkosic however a scour surface is present within the or limey sandstone with <10% feldspar but arenite with 32% K-feldspar and 1% biotite. uppermost part of the Entrada Sandstone in Es- ~35% calcite, as quantified by QEMSCAN The Middle Wanakah Formation sample (2Jw) calante Canyon, and at Ribbon Trail (Fig. 1). In analysis (samples 2Jw, 3Jw, 3Jmt). Stratifica- contains 13% K-feldspar and is a subarkosic ar- western Colorado, the top contact between the tion is horizontally laminated to wavy lami- enite. The upper sample of the Wanakah Forma- Wanakah Formation and the Tidwell Member nated to massive bedding. Bioturbation and tion (3Jw) and the upper Tidwell Member sam- of the Morrison Formation is typically placed salt casts are present. This lithofacies is present ple (3Jmt) are quartz wackes that contain ~30% at the base of the first major sandstone, Marker in <40-cm-thick beds and is typically topped calcite cement and less than 10% K-feldspar. Bed A (Fig. 1). This is a cross-bedded sandstone by the micrite lithofacies. The planar stratified The Lower Tidwell Member sample (2Jmt) is a with distinctive coarse sand chert lags, and the sandstone lithofacies is a fine- to medium- quartz arenite. J-5 unconformity is placed at this boundary grained sandstone. It is present in the Entrada (Fig. 1; Peterson, 1994; O’Sullivan, 2004). Sandstone and the basal Wanakah Formation. INTERPRETATION This lithofacies ranges from a feldspathic ar- Lithofacies enite to a feldspathic wacke based on QEMS- Depositional Environment CAN analysis (samples 1Jeeb, Jeef) Major lithofacies in the Wanakah Forma- A micrite lithofacies and a gypsum litho- The Wanakah Formation exhibits the follow- tion and the Tidwell Member of the Morri- facies are also present. The micrite lithofacies ing diagnostic sedimentary features: (1) lami- son Formation are described next. The major exhibits wavy lamination with alternating light nated and massive (bioturbated) mudstones, lithologies are siliciclastic mudstones, silt- and dark lamina. Beds are <20 cm tall and can (2) paleosols, (3) tabular sandstones with algal stones, and sandstones, but micrite is present be mounded in <10-cm-tall ridges. This litho- mats, (4) a lack of marine , and (5) gyp- in the Wanakah Formation and common in the facies is commonly directly above the wavy sum and salt casts. Laminated and bioturbated Tidwell Member. A gypsum lithofacies is com- sandstone lithofacies. Gypsum beds are only mudstones suggest quiet-water deposition. Pa- mon in the Wanakah Formation, but only in the exposed in the Black Canyon of the Gunnison. leosol features such as root traces, blocky peds, Black Canyon of the Gunnison. The descrip- Beds are between 1 and 3 m thick and are fine- disrupted bedding, and carbonate concretions tion of the uppermost eolian Entrada Sand- grained and horizontally planar laminated or suggest subaerial exposure of the sediments and stone is limited to the uppermost 3 m, which globular and horizontally planar bedded. The periods of nondeposition where soils could de- consist of planar stratification. A stratigraphic beds are commonly interbedded with horizon- velop in a terrestrial setting. Tabular sandstones section of Escalante Canyon illustrates typical tally planar laminated mudstone. One bed in the topped with microbialites (wavy laminated mi-

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A B

CC DD

~10cm EFE F

1m 5cm

Figure 3. Wanakah Formation features. (A) Escalante Canyon (section is 30 m). (B) Duncan Trail in Black Canyon of the Gunnison (trees, ~100 m tall, for scale). Yellow dashed line in A and B separates a regional feature of lower red shale from underlying a green shale. (C) Ten Mile Graben. (D) Ribbon Trail. A black volcanic ash separates the red and green shales in C and D. (E) Prominent wavy sandstone lithofacies (labeled 3 and 4 in Fig. 1). (F) Micritic algal mat that overlies the wavy sandstone. See Figure 2 for locations.

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Figure 4. Age probability charts derived using macros for Excel available from the Arizona LaserChron Center. Red numbers are major age accumulations identified with AgePick software (also from Arizona LaserChron Center). Sample numbers and numbers of individual U/Pb ages for each sample are in black. Numbers of ages represent actual numbers used for statistical analysis with the discarded analyses removed (see Methods section for explanation). Mbr— Member; Fm—Formation; Ss—Sandstone.

crites) are interpreted as shoreline sandstones with transgressive water levels allowing for al- gal precipitation of carbonate. Most depositional environment interpreta- tions of the Wanakah Formation suggest a la- custrine origin, particularly those done on the Todilto Limestone to the south of the study area (Peterson, 1988a, 1994; Turner and Peterson, 1992; Anderson and Lucas, 1992, 1994; Kirk- land et al., 1995; Benan et al., 2000; O’Sullivan, 2004; O’Sullivan et al., 2006; Lucas et al., 2006). In the study area, the lack of marine fos- sils lends support for a lacustrine interpretation, and the overall restricted biodiversity (limited to trace fossils and microbialites) suggests hy- persaline water chemistry. Additional support for a hypersaline lacustrine interpretation is the presence of thick (<3 m) gypsum lenses in the

1% 2% Black Canyon of the Gunnison. A restricted ma- 3% 1% 1% 1% 1% 5% 5% 1% 5% rine environment interpretation is possible, but 11% 10% 15% 10% the development of paleosols and lack of marine 18% fossils favor a terrestrial depositional setting. 9% An arid climate setting is interpreted, due to the 25% 3% 7% presence of gypsum and paleosols and rework- 23% 6% 20% ing of gypsum into sand dune deposits (suggest- ing episodic desiccation).

10% 1% 68% 54% Provenance 9% 10% 2% 38% 39% Detrital zircon U/Pb age populations from the six samples fit within major age categories 30% defined by Dickinson and Gehrels (2003, 2008, 26% 29% 2010), and the spectra for the Entrada Sand- stone and Morrison Formation samples are 5% 13% fairly typical for Colorado Plateau Mesozoic 5% 1% 6% 12% 8% 12% 9% 15% 10% 7% 12% 7% 6% 1% 9% 4% 9% Figure 5. Age populations as distributed in 3% 7% 2% major age categories as defined by Dickinson 1% 8% 5% 1% 5% 2% 1% 5% 3% 1% 2% 4% and Gehrels (2003, 2008, 2010). Note the dra- Jebb Jeef 2Jw3Jw 1Jmt 2Jmt Jms matic increase in Appalachian orogen population beginning with Jeef (inter- McClure Mountain syenite 165-145Ma Cordilleran 2.3-2.0Ga Wopmay Orogeny 1.53-1.3Ga Anorogenic Craton 512-530Ma Backarc preted herein as basal Wanakah Formation) and 1.8-1.55Ga Yavapai-Mazatzal 723-530Ma Neoproterozoic ending in 1Jmt (Marker Bed A of the Tidwell 3.2-2.4Ga Archean Craton 241-181Ma Cordilleran Arc Orogenies Appalachian Orogeny Member). This suggests an unroofing sequence 2.0-1.8Ga 880-750Ma Unknown 504-285Ma Paleozoic as well as a conformable contact between the > 3.2Ga Unknown Suture Belt 1.3-0.9Ga Grenville Orogeny Appalachian Orogeny Wanakah Formation and the Tidwell Member.

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sandstones (Fig. 5; Dickinson and Gehrels, was exposed; or (3) a combination of these two. reworking of the Triassic Santa Rosa Formation 2003, 2008, 2010). The Entrada Sandstone and The following section outlines our interpretation (within the Chinle-Dockum system), which has the Morrison Formation populations exhibit a by discussing the source of the sediment and the a similar detrital zircon signature to that of the majority of grains from Cordilleran back-arc depositional sequences and environments, and it Wanakah Formation at Escalante Canyon with a activity (165–145 Ma), Paleozoic Appalachian ends with a summation of provenance. marked peak at 516 Ma (Dickinson and Gehrels, orogeny (504–285 Ma), Neoproterozoic Ap- 2008, 2010). The Santa Rosa Formation is inter- palachian orogeny (defined by Dickinson and Source of 512–539 Ma Zircons preted to be derived from the Amarillo-Wichita Gehrels as 723–512 Ma, but separated for this Province in Texas and Oklahoma and deposited study), the Grenville orogeny (1.3–0.9 Ga), The McClure Mountain syenite by paleorivers flowing northwest into the Trias- anorogenic craton (1.53–1.3 Ga), is part of a complex of mafic-ultramafic cumu- sic Eagle paleovalley (Dickinson and Gehrels, and the Proterozoic Yavapai-Mazatzal orogeny lates, hornblende-biotite syenites, nepheline 2008, 2010). Additionally, reworking of the (1.8–1.55 Ga; Fig. 5). The increase in 241–181 syenites, and mafic nepheline-clinopyroxene Santa Rosa Formation can also account for the Ma grains in the Morrison Formation samples rocks intruded by carbonatite, several kinds grains in the unknown population range from compared to older samples could reflect an of syenite, and lamprophyre dikes exposed in 750 to 880 Ma present in the Wanakah Forma- increase in tephra air-fall deposition, which is the Wet Mountains area of central Colorado tion (Dickinson and Gehrels, 2008, 2010). abundant as tuffs in the Morrison Formation (Fig. 6; Parker and Hildebrand, 1963; Shawe However, the Santa Rosa Formation is the basal related to Cordilleran volcanism (Turner and and Parker, 1967; Armbrustmacher, 1984). The Chinle-Dockum unit, and subsequently it was Fishman, 1991; Kowallis et al., 1998). U-Pb age of the hornblende-biotite syenite is buried beneath hundreds of meters of younger The most interesting aspect of the detrital 523 ± 0.12 Ma (Schoene and Bowring, 2006). Triassic Chinle-Dockum strata capped by a zircon data is the quantity of grains between This unit would have potentially been exposed continuous blanket of Middle Jurassic Entrada 512 and 539 Ma in the Wanakah Formation in the remnants of the Ancestral Front Range Sandstone that extended all the way to the Okla- samples along with the concomitant decrease during the Middle Jurassic, and the abundance homa panhandle before burial under in Grenville orogeny grains (1300–900 Ma). of K-feldspar in the syenite (Armbrustmacher, strata of the interior seaway (William Dickin- The Grenville orogeny is typically the most 1984; Schoene and Bowring, 2006) could well son, 2015, personal commun.). prevalent source in most Colorado Plateau Me- explain the surge in K-feldspar and the trace Another possibility is a reworking of the sozoic deposits (Dickinson and Gehrels, 2003, amounts of biotite present in the Wanakah For- Temple Butte Sandstone, exposed in 2008, 2010). A gradual increase in the 512–529 mation sediments that are coincident with the northern Arizona and southeast Nevada (Dick- Ma age population begins directly above the input of 512–539 Ma zircons. inson and Gehrels, 2003). This unit exhibits a scour surface in the Entrada Sandstone (Figs. Several other possible sources for the 512– minor population of grains derived from the 4 and 5), with 20% of the grains clustering at 539 Ma grains were considered as well. One is a Amarillo-Wichita uplift (Dickinson and Geh- 519 Ma. This age population dominates the Wanakah Formation sandstones, with >50% of grains clustering at 523–527 Ma (Figs. 4 and 5). The cluster at 523 Ma is less but still present in the basal Tidwell Member (Marker Bed A) sandstone and is absent in the overlying Tidwell Member sample (Figs. 4 and 5).

DISCUSSION

The dramatic increase in 512–539 Ma sedi- ments that begins above the scour surface in the Entrada Sandstone and ends after Marker Bed A of the Tidwell Member suggests the denuda- tion of some unit with an abundance of ca. 512– 539 Ma grains. The source of these 512–539 Ma grains is interpreted herein as the Cambrian intrusives in central Colorado, specifically the McClure Mountain syenite (523.98 ± 0.12 Ma; Bickford et al., 1989; McMillan and McLemore, 2004; Schoene and Bowring, 2006). The abrupt and abundant input of Cambrian grains that ta- pers off through the deposition of the Tidwell Member suggests several possibilities: (1) a late- ~610km stage uplift on the southwest side of the Ancestral Front Range that exposed the McClure Moun- tain syenite, which then eroded into a lacustrine Figure 6. Circa 160 Ma paleogeographic reconstruction using base map from Blakey (2014) and data from this research and Dickinson and Gehrels (2008, basin to the northwest; (2) stream capture that 2010) for interpretation. The McClure Mountain syenite provided much of the diverted a major drainage into the basin from sediment to this basin. Additionally, these units were deposited in a large highlands where the McClure Mountain syenite hypersaline lake system that was episodically desiccated.

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rels, 2003). However, the unit is also buried be- U-Pb ages that accompanied the transition from 6). This lake system and drainage basin likely neath thousands of meters of strata and does not eolian sedimentation of the Entrada Sandstone evolved into the major fluvial system that depos- have the quantity of ca. 525 Ma grains to explain to hypersaline lacustrine sedimentation of the ited the Salt Wash Member of the Morrison For- the amount present in the Wanakah Formation. Wanakah Formation and the Tidwell Member of mation. This study illustrates how U-Pb detrital Therefore, the McClure Mountain syenite the Morrison Formation. The abrupt appearance zircon geochronology can be used to illuminate is the most likely source for these 512–539 of sediments sourced from the McClure syenite, drainage patterns and landscape evolution, not Ma grains because of its proximity to the basin along with the creation of lakes, is consistent just on Laurentia during the Middle Jurassic, and because it was likely exposed during the with the capture of a river system and an accom- but in any basin with similar changes in deposi- Middle Jurassic. Denudation of the McClure panying increase in discharge and water-table tional environments. Mountain syenite could also explain the influx rise in southwest Laurentia (Smith et al., 1989; of the 512–539 Ma zircons and the observed Ashley et al., 2004; Carroll et al., 2008; Moore ACKNOWLEDGMENTS K-feldspar and biotite; neither reworking of and Eckardt, 2012). Funding for this research was provided by a grant from the Unconventional Energy Center for Applied Research at Colo- the Santa Rosa Formation nor reworking of Stream capture and drainage reorganization rado Mesa University and the American Chemical Society Temple Butte Sandstone would explain the can be triggered by a variety of mechanisms, Petroleum Research Fund (both to Potter-McIntyre). We grate- lithological change that is observed. including tectonism, base-level changes, head- fully acknowledge the participation of students at Colorado Mesa University, especially Thomas Spain, for help with field ward erosion, and lake spillover (Brocklehu- research. The manuscript was strengthened with thought- Depositional Environments and rst and Whipple, 2002; Schoenbohm et al., ful reviews by William R. Dickinson, Michael Wagreich, and Stratigraphic Relations 2004; Brook et al., 2006; Clark et al., 2006; Glenn R. Sharman. Oskin and Burbank, 2007; House et al., 2008; The Wanakah Formation (like the Tidwell Crossey et al., 2015). It is possible that a tec- REFERENCES CITED Adler, H., 1974, Concepts of uranium-ore formation in reduc- Member) was likely deposited in a large hy- tonic event simply uplifted the southwestern ing environments in sandstones and other sediments, persaline lacustrine environment that was epi- flank of the Ancestral Front Range, exposed in Formation of Uranium Ore Deposits: Vienna, Inter­ sodically desiccated (as evidenced by paleosols, the syenite, and caused this influx of McClure national Atomic Energy Agency, p. 141–168. Anderson, O.J., and Lucas, S.G., 1992, The Middle Jurassic lack of marine fossils, and thick gypsum depos- Mountain syenite grains; however, uplift alone Summerville Formation, northern New Mexico: New its, some of which were reworked into cross- cannot account for the accompanying shift in Mexico Geology, v. 14, p. 79–92. 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